Covered stents, or stent grafts, have been used to treat a variety of medical conditions, including aneurysms, occluded vessels, and restenosis. For example, an aneurysm may occur in a blood vessel in a location where, due to age, disease, or genetic predisposition, the blood vessel strength or resiliency is insufficient to enable the blood vessel wall to retain its shape as blood flows therethrough. This may result in ballooning or stretching of the blood vessel at the location having limited strength or resiliency, thereby forming an aneurysmal sac. If the aneurysm is left untreated, the blood vessel wall may continue to expand to the point where the remaining strength of the blood vessel wall is insufficient to prevent rupture. In this instance, the blood vessel may fail at the location of the aneurysm, often with fatal result.
To prevent rupture, a stent graft of a tubular construction may be introduced into the blood vessel, for example, intraluminally. Typically, the stent graft is deployed and secured in a location within the blood vessel such that the stent graft spans the aneurysmal sac. The outer surface of the stent graft, at its opposed ends, may be sealed to the interior wall of the blood vessel at locations where the blood vessel wall has not suffered a loss of strength or resiliency. Blood flow in the vessel may be channeled through the hollow interior of the stent graft, thereby reducing, or possibly eliminating, the stress on the blood vessel wall at the location of the aneurysmal sac. The graft material of the stent graft may be substantially nonporous so that blood may be prevented from leaking into the aneurysmal sac. This may reduce the risk of rupture of the blood vessel wall at the location of the aneurysmal sac while allowing blood to continue to flow through the stent graft to the downstream blood vessels without interruption.
Various materials and methods have been used to create coverings, or grafts, that may be applied to stents to form stent grafts. For example, grafts may be made using woven techniques, thin-sheet/tubing bonding, or other processes. Electrospinning may be used to form a suitable biocompatible coating or covering for a medical device, such as a stent graft. Electrospinning is a process for creating a nonwoven network of fibers using an electrically charged solution that is driven from a source to a target with an electrical field. More specifically, a solution is driven from an orifice, such as a needle. A voltage is applied to the orifice resulting in a charged solution jet or stream from the orifice to the target. The jet forms a conical shape, termed a Taylor cone, as it travels from the orifice. As the distance from the orifice increases, the cone becomes stretched until the jet splits or splays into many fibers prior to reaching the target. The fibers are extremely thin, typically in the nanometer range. The collection of fibers on the target forms a thin mesh layer of fibrous material.